This invention relates to tip treatment bars of a rotor casing for a gas turbine engine.
WO94/20759 discloses an anti-stall tip treatment means in a gas turbine engine, in which an annular cavity is provided adjacent the blade tips of a compressor rotor. The cavity communicates with the gas flow path through the compressor past a series of bars extending across the mouth of the cavity.
Such tip treatments are applicable to both fans and compressors of gas turbine engines, and their purpose is to improve the blade stall characteristics or surge characteristics of the compressor.
The passage of the blade tips past the bars creates vibrations in the bars which, over time, can result in high cycle fatigue failure of the bars. This failure is caused by vibration resonance between the tip treatment bars and the natural engine order modes.
It is an object of the present invention to reduce or eliminate high cycle fatigue failure in tip treatment bars of a gas turbine engine.
It is a further object of the present invention to reduce or eliminate vibration resonance between tip treatment bars of a gas turbine engine and natural engine order modes.
It is a yet further object of the present invention to isolate tip treatment bars of a gas turbine engine from end supports by which they are supported.
It is a yet further object of the present invention to provide damping means between the tip treatment bars and the end supports.
According to one aspect of the present invention, there is provided a gas turbine engine casing comprising tip treatment bars extending between annular end supports, each tip treatment bar being supported at each end by the end supports and being isolated, at at least one end, from the respective end support by damping means.
In a preferred embodiment in accordance with the present invention, both ends of each tip treatment bar are isolated by damping means from the respective end support. The damping means may comprise a damping material, and preferably a damping material having a high degree of damping at higher frequencies (i.e. frequencies in excess of 1000 Hz).
The damping may be a polymer, and preferably an elastomeric polymer. Silicone elastomers may be used, for example the silicone elastomer available under the name SILASTIC J.
Preferably, the tip treatment bars or the end supports, or both, are bonded to the damping material, for example by means of a silicone adhesive. A suitable adhesive is that available under the name SILCOSET 152.
The damping material may comprise a moulded component which is assembled, after manufacture, with the tip treatment bars and the end supports. Alternatively, the damping material may be moulded in situ as the tip treatment bars are fitted to the end supports.
The bars may be solid, but they may alternatively be hollow, or provided with pockets, to lighten the structure. The bars may be made from any suitable material, for example from alloys commonly used in the aircraft industry. In a preferred embodiment, the tip treatment bars and, preferably, the end supports are made from a composite material such as a carbon fibre/bismaleimide composite.